Aerocapture and aerobraking at Pluto. Wow, I'd be damned! At 1/10 000 of Earth sea level atmospheric pressure on Pluto's surface. Still, aerobraking and aerocapture has not really been used, has it? Aerobraking has been tried out several times but has not been a critical part of a mission AFAIK. It looks promising for missions to the 10 known atmospheric bodies in the Solar System and their hundreds of moons. If Pluto has an "atmosphere" useful for aerobraking, then maybe Eris does too.

Is it sure that Pluto's atmosphere is useful for entering orbit? It is a NIAC award so I don't know whether it is a given or the thing to be investigated.

Does it say "aerocapture"? The article is unclear on that. I have a hard time believing that it is possible to get to Pluto and land without some kind of propulsive braking.

No, this is a landed mission that takes advantage of two key facts: Pluto has very low surface gravity and it has a very thin but extended atmosphere. If you employ a very, very large heat shield, you can use very thin upper atmosphere to slow down an entering vehicle with very little heating. There were proposals to have large inflatable re-entry craft that astronauts could use for emergency return to Earth.

This Pluto proposal would use a large inflated balloon and the friction with the thin but extended atmosphere to slow down enough that a small lander could then finish the landing with landing rockets. Because Pluto's gravity is thin, it could then hop tens or hundreds kilometers away to explore several other locations.

Since the only goal of the balloon is to provide a really large surface area, there's no need for complicated guidance and maneuver capability like there is with aerocapture.

I believe that the same trick would work with Triton. I don't believe that this approach would work for any other bodies -- the thin atmosphere doesn't extended far enough vertically because the planet's gravity is too high.

A couple of questions I would have would be whether or not the small lander could carry enough fuel to boost itself into orbit for a planetary reconnaissance. The public information on the proposal emphasizes that the lander must be small so that the balloon only has to slow a small mass.

Does it say "aerocapture"? The article is unclear on that. I have a hard time believing that it is possible to get to Pluto and land without some kind of propulsive braking.

No, this is a landed mission that takes advantage of two key facts: Pluto has very low surface gravity and it has a very thin but extended atmosphere. If you employ a very, very large heat shield, you can use very thin upper atmosphere to slow down an entering vehicle with very little heating. There were proposals to have large inflatable re-entry craft that astronauts could use for emergency return to Earth.

Okay, but let's be clear: this is slowing down and landing from ORBIT, not using aerocapture to get into orbit in the first place. The proposal assumes that the spacecraft is already orbiting Pluto, correct?

Does it say "aerocapture"? The article is unclear on that. I have a hard time believing that it is possible to get to Pluto and land without some kind of propulsive braking.

No, this is a landed mission that takes advantage of two key facts: Pluto has very low surface gravity and it has a very thin but extended atmosphere. If you employ a very, very large heat shield, you can use very thin upper atmosphere to slow down an entering vehicle with very little heating. There were proposals to have large inflatable re-entry craft that astronauts could use for emergency return to Earth.

Okay, but let's be clear: this is slowing down and landing from ORBIT, not using aerocapture to get into orbit in the first place. The proposal assumes that the spacecraft is already orbiting Pluto, correct?

No, the large surface area of the balloon plus thin extended atmosphere allows no propulsive* slow down from interplanetary transfer speeds. No orbiting, no aerocapture. This is a very clever idea that makes use of past work on very, very large re-entry shields. Make your shield large enough relative to mass, and the heating is very gentle. With this approach, you can travel to Pluto at New Horizon speeds without carrying all the fuel needed to decelerate to enter orbit or do a direct landing. I think this is a genius idea, although we will have to see if all those annoying engineering details work out.

Return to Pluto? Scientists Push for New Mission to Outer Solar System

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A grassroots movement seeks to build momentum for a second NASA mission to the outer solar system, a generation after a similar effort helped give rise to the first one.

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Nearly three dozen scientists have drafted letters in support of a potential return mission to Pluto or to another destination in the Kuiper Belt, the ring of icy bodies beyond Neptune's orbit, Singer told Space.com.

These letters have been sent to NASA planetary science chief Jim Green, as well as to the chairs of several committees that advise the agency, she added.

"We need the community to realize that people are interested," Singer said. "We need the community to realize that there are important, unmet goals. And we need the community to realize that this should have a spot somewhere in the Decadal Survey."

That would be the Planetary Science Decadal Survey, a report published by the National Academy of Sciences that lays out the nation's top exploration priorities for the coming decade.

"This is the way it normally works," said New Horizons principal investigator Alan Stern, who's also based at SwRI.

"First it bubbles up in the community and then, when there's enough action, the agency starts to get behind it," Stern, who has been the driving force behind New Horizons since the very beginning, told Space.com. "Then it lets the Decadal Survey sort things out."

Stern contributed a letter to the new campaign, and he has voiced support for a dedicated Pluto orbiter. Singer would also be happy if NASA went back to the dwarf planet.

"Pluto just has so much going on," she said.

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"I would say 25 years is the longest I think about," she said, referring to how long it may be before another Kuiper Belt mission gets to its destination. "And I hope it may be more like 15 years."

These slides show the alignments, and the probable trajectories, a flyby mission could undertake during the ~next quarter-century. Jupiter alone could probably fling a probe to most objects, but an alignment with one of the other gas giants enhances the ability to tweak the trajectory...

Neptune could send a probe to any of six objects, the largest being Eris itself (so long as you're willing to wait to reach it). Uranus is sparser with only three, Varuna the major one. A route via Saturn would be as rich as Neptune, and includes the 2 next most massive dwarfs, Haumea and Makemake. This raises a large number of possibilities, all of which depend on the would-be-mission's priorities. For instance....

If the priority is the Kuiper object, either Saturn or Neptune are your best options because either can give you access to the more prominent bodies; Eris would be awesome to see although it'd probably be a tertiary choice since the next-largest-bodies, including the multi-mooned and uniquely-shaped Haumea, are far more accessible in a quicker time scale.

If the priority is Neptune, you get a great chance for fresh science and, thanks to Neptune's position at the edge with less interference from the Sun's gravity well, a wide range to redirect the probe afterwards. Studying Triton against Pluto or the Kuiper belt would be great for comparison. Not as much science as an orbiter naturally, but you get a chance to study both the planet and bodies it affected during its evolution.

If the priority is Uranus or Saturn, there isn't quite as much useful science you could do as compared to the first two. The best chance would be to drop off a Saturn probe, using the Kuiper probe as the carrier. Saturn's main gain would be gaining knowledge about its atmosphere and accessing a great selection of dwarfs, whereas Uranus is likewise its atmosphere, structure, and mapping more of its moons.

Between this and the study summary from Amy Simon and Mark Hofstadter, I'd definitely cross Uranus off the fly-by list and only reserve that as a Neptune option. It is a bonus learning that missions to both Neptune and Saturn could vastly increase our knowledge of the Kuiper Belt as much as New Horizons; I'd definitely like to see the would-be-carrier of a Saturn probe (like ESA's proposed Hera for instance) have an option to fly-by say Haumea afterwards.

Does it say "aerocapture"? The article is unclear on that. I have a hard time believing that it is possible to get to Pluto and land without some kind of propulsive braking.

No, this is a landed mission that takes advantage of two key facts: Pluto has very low surface gravity and it has a very thin but extended atmosphere. If you employ a very, very large heat shield, you can use very thin upper atmosphere to slow down an entering vehicle with very little heating. There were proposals to have large inflatable re-entry craft that astronauts could use for emergency return to Earth.

This Pluto proposal would use a large inflated balloon and the friction with the thin but extended atmosphere to slow down enough that a small lander could then finish the landing with landing rockets. Because Pluto's gravity is thin, it could then hop tens or hundreds kilometers away to explore several other locations.

Since the only goal of the balloon is to provide a really large surface area, there's no need for complicated guidance and maneuver capability like there is with aerocapture.

I believe that the same trick would work with Triton. I don't believe that this approach would work for any other bodies -- the thin atmosphere doesn't extended far enough vertically because the planet's gravity is too high.

A couple of questions I would have would be whether or not the small lander could carry enough fuel to boost itself into orbit for a planetary reconnaissance. The public information on the proposal emphasizes that the lander must be small so that the balloon only has to slow a small mass.

I wonder whether you could use this technique to aerobrake at a comet. The density of the coma is much lower than even the atmosphere of Pluto, but it's also much deeper.

Obviously this would be useful for missions to the comet itself, but if you could find a comet travelling in roughly the right direction, you could hitch a ride and use the comet to provide much of the velocity needed to reach the outer solar system.

More on this, it appears they have worked out a way to use Charon to slingshot a Pluto orbiter around the system and eventually out into the Kuiper Belt for an extended mission to other dwarf planets, essentially getting a two-for-one.

A Southwest Research Institute team using internal research funds has made several discoveries that expand the range and value of a future Pluto orbiter mission. The breakthroughs define a fuel-saving orbital tour and demonstrate that an orbiter can continue exploration in the Kuiper Belt after surveying Pluto. These and other results from the study will be reported this week at a workshop on future Pluto and Kuiper Belt exploration at the American Astronomical Society’s Division for Planetary Sciences meeting in Knoxville, Tennessee.

But it’s also the move into the Kuiper Belt that has Stern’s attention. It makes a Pluto orbiter of this design a multi-purpose spacecraft and leverages our growing experience with ion propulsion. Says Stern:

“This is groundbreaking. Previously, NASA and the planetary science community thought the next step in Kuiper Belt exploration would be to choose between ‘going deep’ in the study of Pluto and its moons or ‘going broad’ by examining smaller Kuiper Belt objects and another dwarf planet for comparison to Pluto. The planetary science community debated which was the right next step. Our studies show you can do both in a single mission: it’s a game changer.”

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From the paper, this is a bit of an eye-opener:

There is a KBO mission possible for every Earth-Jupiter launch window throughout a Jupiter revolution, thus Pluto and every one of the selected 45 KBOs are accessible via Jupiter gravity assist with a flight time of under 25 years and a C3 [excess launch energy] less than 140 km2/s2. Many, but not all objects can be reached via Saturn flyby, and a smaller list still can be compatible with a visit to an ice giant, though it does not necessarily provide a TOF [time of flight] advantage.

Which leads to this:

We found that all five of the non-Pluto KBOs studied by McGranaghan et al [23] can be reached by giant planet swingby— (136199) Eris and (90377) Sedna with Neptune, and (50000) Quaoar, (136472) Makemake and (136108) Haumea via Jupiter-Saturn. Fast-rotator (20000) Varuna is reachable after a Uranus encounter.

Not surprised they realized Charon could be useful for a gravity assist; in some Uranus orbiter studies they likewise concluded even Uranus' relatively puny moons could be relatively as useful as the Galileans/Titan despite being planetary lightweights. Good to calculate ahead of time, although getting into orbit around Pluto is the far greater challenge I'd like to hear a solution to.